The glasses already known in this field of use are numerous and mostly belong to the large family of the aluminoborosilicate glasses, which include the alkaline earth oxide glasses.
These glasses are chosen according to the physico-chemical properties which the substrate should possess. In fact, where the glass substrate has on its surface a network of thin film transistors, this film is obtained after several operations, during which the glass has been subjected to relatively high temperatures. The glass must not deform during these operations and should therefore have the highest possible strain point. Furthermore, the coefficient of thermal expansion of the glass forming the substrate must be compatible with that of the films which are deposited on its surface. Finally, the glass must virtually contain no elements that are likely to migrate into the films formed and to cause the network properties of the transistors to deteriorate; this is the case, in particular, for the alkalines.
These glasses also should have viscosity and devitrification characteristics that are adapted to the process chosen for producing a glass sheet, the thickness and the surface of which shall be as uniform as possible.
These glasses must also have good chemical resistance with respect to acid media, notably with respect to solutions based upon hydrofluoric acid.
One of the processes that may be used consists of bringing the molten glass into a device, the top of the side walls of which, converging downwards, act as a spillway. The glass flows along the side walls, thus forming two currents which join at the apex of the device, before being drawn vertically downwards in the form of a plane sheet. Such a process requires that the glass shall have, at the liquidus temperature, a viscosity equal to at least 2 to 3.times.10.sup.5 poises. Such glasses are, for example, illustrated in the patent U.S. Pat. No. 4,824,808.
Another process consists of spreading the molten glass over a bath of molten tin in accordance with the well known float glass technique. Glass compositions that are capable of being floated and intended for use as substrates for forming flat screens are described, notably, in patent application Ser No. WO 89/02877.
These glasses belong to the family of the alumino-borosilicates, having a high content of alkaline earth oxides. Whereas these glasses possess good pbysico-chemical characteristics having regard to the application under consideration, their composition contains a high percentage of expensive oxides, such as B.sub.2 O.sub.3, SrO and BaO. Furthermore, they are fairly viscous for the float glass process, if judged from the examples illustrating that invention. In fact, the temperature corresponding to log.eta.=4 is equal to or greater than 1,150.degree. C., even higher than 1,200.degree. C. It may be remembered that the temperature corresponding to the same viscosity for an ordinary float glass (silico-sodo-calcic glass) lies between 1,000.degree. and 1,050.degree. C.